In many types of motor operated control systems, and particularly in temperature responsive systems, the condition responsive element is an off-on type of condition sensor. Many condition responsive systems require a modulating type of control action in order to maintain the desired control function. Obtaining a modulating action from a normally on-off type of condition sensor, such as a thermostat, is rather complex and expensive. An off-on type of condition sensor can be used with a heat motor type of actuator, which has a long time constant of operation, to provide a modulating-like function. A heat motor actuator, however, ordinarily does not have sufficient power to operate a substantial load without becoming excessively costly.